Project 3.2

Improving historical estimates of abundance and distribution of dugongs and large green turtles in western and central Torres Strait

Led by: Prof Helene Marsh, JCU

Project Summary

This project will improve the accuracy of aerial survey estimates of the size and distribution of dugong and green turtle populations in western and central Torres Strait by: (1) collecting and analysing data on their movements and diving behaviour, and (2) estimating the proportion of turtles seen during aerial surveys that are the large female green turtles harvested by Indigenous hunters. These data will then be used to (re)estimate the size and distribution of the populations of dugongs and harvestable green turtles in Torres Strait by (re) analysing historical aerial survey data collected in 2001, 2005, 2006, 2011 and 2013.

Problem Statements

Problem

Accurate estimates of the population size and distribution of dugongs and green turtles in Torres Strait are important for the Indigenous management of these species.

JCU has conducted dugong aerial surveys since 1987 and has also recorded turtle sightings. As proof of concept, we recently analysed aerial turtle sightings from 2013 using green turtle dive times and turtle ‘secchi disk’ experiments from outside Torres Strait to estimate the detection zones in which turtles are visible from survey aircraft under various environmental conditions. The project team estimated there were ~400,000 adult-sized green turtles in central and western Torres Strait (Fuentes et al. in press). The project aims to improve accuracy by quantifying the proportion of Torres Strait turtles that are adult-sized green turtles.

The team have also discovered that the availability of dugongs to aerial observers depends on bathymetry as well as environmental conditions. Dugongs in waters 5-12 m deep (the depths most Torres Strait dugongs and green turtles are sighted) are less available to aerial observers than animals in shallower or deeper waters.

How Research Addresses Problem

This project will improve availability estimates by collecting dive and movement data from dugong and green turtles living in Torres Strait, in turn improving information on their population sizes and distributions.